What Is Coral Bleaching?

Coral bleaching is a term used to describe what happens when coral loses its endosymbiotic algae. This algae is an important component to the existence of coral. In fact, it provides 90% of the energy coral needs to survive. Once the algae is lost, the coral begins to starve as it is unable to obtain necessary nutrients. Instances of coral bleaching have become more common over the last decade as global climate change continues to cause an increase in the temperatures of the ocean. In 2016, 90% of the Great Barrier Reef experienced coral bleaching, which resulted in a 20% loss of the coral reefs.

What Causes Coral Bleaching?

As previously mentioned, global climate change is believed to be responsible for the increasing number of coral bleaching events. This is because coral bleaching takes place when coral is no longer able to maintain a healthy environment for zooxanthellae, which are single-celled organisms that contribute to the color of coral reef. High water temperatures may affect the ability of coral to produce carbon dioxide and ammonia, which the zooxanthellae needs to create photosynthesis. When this happens, the zooxanthellae falls from the coral reef. This is detrimental to the health of the coral because the algae produces much-needed oxygen and nutrients that the coral relies on for survival.

Some researchers believe that coral bleaching is caused by irradiance induced stress, which is, in turn, caused by high temperatures, low temperatures, toxic chemicals, freshwater, increased solar radiation, and sediment. Coral reacts to this stress by expelling the important algae, leading to bleaching.

What Are The Effects of Coral Bleaching?

When algae is expelled from coral, the coral loses its color and begins to starve. This additional stressor results in decreased reproduction and growth. In this weakened state, coral becomes more susceptible to diseases. If the environmental conditions surrounding the coral improve, the algae may return to its host, in which case, the coral would regain its health. However, if the bleaching event lasts for a long period of time, it is likely that the coral will die.

Places Where Coral Bleaching Has Occurred

Between 1979 and 1990, researchers have recorded at least 60 episodes of coral bleaching in the world's oceans. From 1919 until 2016, coral bleaching has become more frequent and has lasted for longer periods of time. In fact, 2016 saw the longest episode of coral bleaching ever recorded.

One of the most well-known coral reefs in the world, the Great Barrier Reef, has experienced several episodes of coral bleaching over the years. The Great Barrier Reef has experienced bleaching in the following years: 1980, 1982, 1992, 1994, 1998, 2002, 2006, and 2016. The worst of these episodes occurred in 1998 when 42% of the reef was bleached and in 2002 when bleaching spread over 54% of the Great Barrier Reef. Between 1985 and 2012, the Great Barrier Reef Lost 50.7% of its coral population. Researchers estimate that 10% of this loss was due to coral bleaching.

Other coral bleaching events have occurred off the coasts of Tanzania, Kenya, the Maldives, Sri Lanka, and the Seychelles. In total, these coastlines have lost around 90% of their coral reef populations. The US state of Hawaii has also experienced coral bleaching with severe events occurring in 1996, 2002, and 2014.